The present invention relates to a method for braking of a motor vehicle, which is driven by a drive torque from at least one electrical machine. The present invention also relates to a motor vehicle.
Electric or hybrid vehicles, which are driven by at least one electrical machine, place new demands on the braking system of the motor vehicles. In these motor vehicles, the electrical machine can be operated as a generator for braking of the motor vehicle. With this so-called regenerative braking, a portion of the kinetic energy of the motor vehicle can be recovered as electrical energy in a braking operation.
Since the braking torques of the electrical machines are usually insufficient to allow full braking of the motor vehicle, friction brakes are also installed, with which the required braking torque can be generated. However, it is advantageous for energy efficiency to convert with the electrical machine as much of the kinetic energy of the motor vehicle as possible into electrical energy and to dissipate the smallest possible portion as heat with the friction brakes.
When the driver operates the brake pedal of the friction brake, he must initially overcome with the pedal a free travel where the friction brake does not generate a braking force. To recover the greatest possible amount of the kinetic energy with the electric motor or the electrical machine, recuperation must take place already in the free travel of the brake pedal. However, no energy can be fed back into the battery from the electrical machine operating in generator mode when the battery or another energy storage device of the motor vehicle is fully charged. Hence no braking effect can be produced by the electrical machine, changing the braking force acting on the vehicle. This change in braking performance may surprise and unsettle the driver.
To limit this effect on the braking performance, the braking torque of the electrical machine is limited in electric or hybrid vehicles having a conventional brake system. However, the potential of the recuperation cannot be fully exploited as a result. Furthermore, so-called blending-enabled braking systems are used, which activate the friction brake regardless of the operation of the brake pedal by the driver so as to provide most uniform braking behavior. However, these braking systems are expensive. Moreover, solution approaches are used wherein with a fully charged battery the energy of the electrical machine is dissipated as heat via an additional ohmic resistor. However, this approach disadvantageously requires additional space, and additional costs are incurred due to the installation of the additional resistor.
DE 103 36743 A1 describes a system which includes a control unit for a drive train for a coordinated torque control of a motor vehicle, in particular of an electric vehicle or a hybrid vehicle. The torque provided by the individual torque sources is hereby controlled, or a reduction of the torque, for example by flywheels or by counter-current brakes. Here, the operating parameters of the electrical machine or the state of charge of the battery are also monitored. Furthermore, the pedal feedback and the gear shift of the vehicle can be adjusted depending on the driving situation and the driving style of the driver.